The present invention relates to processes for separating and purifying perfluorinated products from a starting mixture containing a variety of compounds by using azeotropic and extractive distillation processes, such that perfluorinated products of high purity are obtained.
Various gaseous fluorine-containing compounds are utilized in manufacturing processes that plasma-etch silicon-type materials in order to fabricate semiconductor devices. A major use of tetrafluoromethane (CF4 or PFC-14) is for plasma etching during semiconductor device fabrication. Plasma etchants interact with the surface of the integrated circuit wafer, modifying it so as to lay down the electrical pathways and providing for the surface functionalities that define the integrated surface. A major use of nitrogen trifluoride (NF3) is as a xe2x80x9cchemical vapor depositionxe2x80x9d (CVD) chamber cleaning gas in semiconductor device manufacture. CVD chamber cleaning gases are used to form plasmas which interact with the internal surfaces of semiconductor fabrication equipment to remove the various deposits that accumulate over time.
Perfluorinated chemicals such as PFC-14 and NF3 that are used in semiconductor manufacturing applications as etchant or cleaning gases are more commonly referred to as xe2x80x9celectronic gasesxe2x80x9d. Electronic gases having high purity are critical for such semiconductor device manufacture applications. It has been found that even very small amounts of impurities in these gases that enter semiconductor device manufacturing tools can result in wide line width and thus less information per device. Moreover, the presence of these impurities, including but not limited to particulates, metals, moisture, and other halocarbons in the plasma etchant or cleaning gases, even when only present in the part-per-million level, increases the defect rate in the production of these high-density integrated circuits. As a result, there has been increasing demand for higher purity etchant and cleaning gases, and an increasing market value for the materials having the required purity. Identification of offending components and methods for their removal consequently represents a significant aspect of preparing the fluorine-containing compounds for these applications.
These etchant and cleaning gases are not fully consumed by semiconductor manufacturing processes, but typically exit the integrated circuit fabrication equipment in finite concentrations. These fabrication equipment exhaust streams not only contain varying amounts of the unreacted perfluorinated etchant and cleaning gases, but may also contain a variety of reaction products and air components, which include but are not limited to hydrogen fluoride (HF), tetrafluoroethylene (C2F4 or PFC-1114), methyl fluoride (CH3F or HFC-41), trifluoromethane (CHF3 or HFC-23), chlorotrifluoromethane (CClF3 or CFC-13), nitrogen, oxygen, carbon dioxide, water, methane, ethane, propane and nitrous oxide (N2O). A variety of other fluorinated compounds are also used in semiconductor manufacturing applications, including hexafluoroethane (C2F6 or PFC-116), octafluorocyclobutane (cyclic C4F8 or PFC-C318), octafluoropropane (C3F8 or PFC-218), sulfur hexafluoride (SF6), pentafluoroethane (C2HF5 or HFC-125), trifluoromethane (CHF3 or HFC-23), tetrafluoroethane (C2H2F4, or HFC-134a or HFC-134) and difluoromethane (CH2F2 or HFC-32), and the exhaust streams coming off these processes are frequently combined with the exhaust streams from the PFC-14 and NF3 processes. The resulting combined exhaust stream consequently may contain a wide range of compounds and concentrations.
Considerable effort is underway to develop ways and means to capture the fluorinated compounds present in such equipment exhaust streams and to develop options for their disposition. A preferred disposition option is to repurify certain of the fluorinated components from these streams for reuse. Separation of several of these valuable fluorinated compounds is made difficult due to the variety of fluorinated compounds that might be present in the combined exhaust gas stream from any given manufacturing site, and due to non-ideal interactions that exist between several of these compounds. For example, several of these compounds form azeotropic or azeotrope-like compositions with other compounds in these streams, making separation by conventional distillation at least difficult, if not impossible.
The present invention provides compositions and distillation processes for removing fluorinated impurities from an impure stream comprising at least one of PFC-14 and NF3 so as to produce a purified PFC-14 and/or NF3 product. The present processes are simple to carry out and are effective for obtaining either of these two compounds in high purity and with high degrees of recovery.
The present invention comprises NF3 substantially free of impurities, containing less than 10 parts-per-million molar of impurities. The present invention further comprises NF3 containing less than 10 parts-per-million molar PFC-14.
The present invention further comprises azeotropic compositions consisting essentially of: NF3 and PFC-14; hydrogen chloride and PFC-14; NF3 and hydrogen chloride; nitrous oxide and trifluoromethane; and nitrous oxide and hydrogen chloride.
The present invention further comprises a process for separating at least one of PFC-14 and NF3 from a first mixture comprising PFC-14, NF3, and optionally other fluorinated compounds, comprising the steps of:
contacting at least one entraining agent with the first mixture to form a second mixture,
distilling the second mixture, and
recovering at least one of PFC-14 and NF3 that is substantially free of at least one of the other fluorinated components of the first mixture.